I want my thesis to be the best it possibly can be. My supervisor is away for one month right before my thesis submission. Is it inappropriate or poor etiquette to ask another professor in the field who has shown interest in the topic to read the paper and offer any suggestions? This other professor teaches at a different institution, and will not be part of the panel or defence process.

I would ask for help from another academic at your institution, but as soon as you circulate your work to anybody external you are at risk of breaking the university's rules on copyright/IP.
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PhilJul 25 '14 at 14:08

I would ask the other professor, but I would phrase it in a way that makes it clear that I'm not expecting them to proofread the entire thesis. Perhaps you could say something like "You expressed an interest in my research, so I wanted to give you a draft of my thesis. Perhaps it might be of interest to you or one of your students. And of course, If you have any comments or suggestions, I'd be glad to hear them." They probably will read it and give you some comments, but they won't feel under pressure to do so.

This strategy might avoid discomfort if the potential reviewer declines, but it also feels vaguely misleading, or at least unnecessarily indirect. Asking for what you want clearly and politely should always be acceptable, and I would suggest the reviewer will be more likely to outright decline as opposed to forgetting or doing a partial job, which is a good thing.
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TimJul 24 '14 at 20:32

5

If you phrase it like that, the professor will think he's being offered the opportunity to read it and comment, rather than being requested to read it and comment. Something like "I'm sure you must be really busy but if you had time to read this and give me some feedback before I hand it in on date X, I'd be really grateful" makes it clear that you are asking for something, while still giving the professor the chance to say no.
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David RicherbyJul 24 '14 at 22:25

If the other professor has expressed his interest in reading your thesis and is willing to do it, I don't see why not. It is better to have someone read your work out of interest rather than obligation. But it is also a good idea to let your supervisor know about it.

As a student I did not tell my supervisor that I was showing my thesis to someone else because I knew he would not care to know and would have encouraged me to do so (to get comments, ideas, ...). He was an excellent supervisor: present and active when I needed him and siding away when I was moving full speed ahead, sometimes against his recommendations. When one of these "without his recommendation" ("against" is maybe too strong a word) turned out to be a good idea he wrote himself off the paper I was publishing (he was second author) because he said he was not convinced at the time and that I should get all the praise. I am glad I met him.

I was also asked several times to have a look at a thesis. To be frank I never thought of asking the student whether his supervisor was OK or not with that because I did not think about the possibility of being against (and the associated reason).